Tens of thousands of homeowners with home-equity lines of credit are getting a rude surprise: They’ve been told by their lender that they no longer can take money out on their credit lines because sinking home prices have left them with little or no equity.

Among the lenders taking such action is Countrywide Financial Corp., which sent letters to 122,000 customers last week telling them they no longer could borrow against their credit lines. In some cases, the company says, the borrowers are “upside down” on their mortgages — the total debt on the home exceeds the property’s market value.

Countrywide says it is using computer modeling that factors in changes in home prices to determine which customers would have their cash spigot shut off.

The cutoffs are coming as a shock to some.

“We didn’t deserve this,” Thaleia Georgiades, a real estate agent in El Dorado, Calif., said Thursday, two days after she and her husband, a builder, received a letter saying their Countrywide credit line had been frozen. “When you are self-employed, that’s the money you count on to bridge the gap during tough times. And this is a particularly tough time in both the building and housing industries.”

The move by Countrywide, the nation’s largest mortgage company, is part of a pullback by lenders nationwide on home-equity loans, which are often used to finance home improvements and consumer spending. Such loans were widely available until eight months ago, when delinquencies and foreclosures began to soar. Now, with new evidence of sinking home values, many lenders are requiring that homeowners maintain a much larger percentage of home equity as a cushion against financial problems.

Among the lenders tightening as the Federal Reserve loosens interest rates, Chase Home Lending, which has been raising credit standards since last summer, started imposing new guidelines in February that further restrict who can get a home-equity line, the company said.

The new percentages will be even lower for people who don’t have the best credit.

“Our goal is to always make sure that for both our sake and our customer’s sake that our customers don’t owe more than their equity,” said Thomas Kelly, a spokesman for Chase, a unit of banking giant JP Morgan Chase & Co.

Chase is still assessing whether to rescind existing lines of credit, he said.

Falling home prices are also affecting how first mortgages are being made. Fannie Mae, the government-sponsored mortgage investor, has told lenders that in areas that have had significant price declines, the company will require a lower maximum “loan-to-value” ratio on loans it buys.

Just a decade ago, few lenders would extend credit totaling more than 80 percent of a home’s value, and the industry appears to be headed in that direction, said Guy D. Cecala, publisher of Inside Mortgage Finance Publications in Bethesda, Md.

A home-equity loan that puts the total debt on a home over that level can be risky, especially for the lender. That’s because if the property goes into foreclosure, Cecala said, “All the money goes to pay off the first mortgage holder.”

In fact, in cases of foreclosure, home-equity loans are being sold to adventurous investors for as little as 3 percent of the amount of the loan, said Paul Muolo, data editor for National Mortgage News.

Many lenders, including Citigroup Inc. and JPMorgan Chase, reported lower fourth-quarter earnings because of losses on home-equity credit lines.

San Francisco-based Wells Fargo & Co., which avoided many of the mistakes in mortgage securities and subprime loans that have plagued rivals, recently added $1.4 billion to its provision for loan losses, mainly on home equity loans.

Wells Fargo executives “did not fully appreciate the severity of the residential real estate downturn and its impact on our home-equity portfolio,” Mike Loughlin, chief credit officer at Wells, said in a statement released with the bank’s financial results.

Some small lenders also have been forced to change their policies in response to the downturn in home prices. Cityside Federal Credit Union in Los Angeles, which has 6,482 members and $53 million in assets, has reduced its maximum loan-to-value ratio to 90 percent from 100 percent and is rejecting many requests from members who had hoped to refinance mortgages at lower rates.

“A lot of people are in that situation right now because they just kept refinancing and took out all the equity in their homes,” said Teresa Becerra, a senior real estate loan officer at Cityside. “They’ll have a couple of mortgages they want to combine now. But we can do a computer appraisal right away, and what they’re finding out is that the value just isn’t there.”

Cityside has had to cut off some home equity lines of credit as well as turn down borrowers seeking new loans because of the change, Becerra said. So far, it has cut off credit only when customers have brought up issues that cause the credit union to examine their circumstances.

“If they had home equity, and now they’re upside down and it’s brought to our attention, we’ll definitely cut them off,” she said. “But we’re not doing a computer-generated study to find all those situations or anything like that.”