NEW YORK(CBSNewYork) — There was a new warning Wednesday, from the FBI about a growing online scam.

Hackers are hijacking computers and holding them hostage, to get control of the computer back, victims have been forced to pay up.

As CBS2’s Valerie Castro reported, the FBI said the popular scam has targeted hundreds of thousands of victims.

Leah Miller couldn’t believe it when her entire computer froze.

“Whoever hacked my computer had full access,” she said.

Hackers locked all her files. She then received a pop up ransom not demanding payment to free her computer.

It even came with a countdown clock.

“It said ‘pay this ransom before this countdown ends, or you’re going to lose all these files forever,'” she said.

The pop-up that Miller received is called cryptolocker, but the scam is global and the notes come in different forms.

It’s called ransomware, and once the screen pops up you can’t get past it.

“You’re stuck, this is it, you can’t get any further until you follow the hackers instructions to pay them the ransom,” cyber security expert, Scott Schober said.

Schober, of Metuchen, said the ransom demands range from $200 to $1,000, and the criminals often want to be paid in the untraceable online currency Bitcoin.

“It’s very hard to catch the hackers,” Schober said.

It’s not just personal computers, businesses, school districts, even police departments have been hit with the uncrackable code.

Police in Swansea, Massachusetts handed over $750 to set their system free.

“It’s scary,” Miller said.

She refused to pay the $800 ransom and lost all of her files.

A technician wiped her computer clean so she could start over.

“I lost a lot of data and a sense of security,” she said.

She has taken measures to protect herself, experts said it’s important to update anti-virus software and back-up all of your data.

Miller worries that data could return.

“It happened out of nowhere, and I don’t know if and when it will happen again,” she said.

The Cyber Threat Alliance said more than $325-million in computer ransom demands has been paid out globally.