A page popped up, locking the computer, and giving information on how to unlock it for a fee.
Sounds more like a fake ransomware scam.
Actual ransomware usually will have obvious indications (signs of infection)...it typically targets and encrypts data files so you cannot open them on your computer (and all connected drives at the time of infection), in most cases it appends an obvious extension to the end or beginning of encrypted filenames (although some variants do not), demands a ransom payment by dropping ransom notes in every directory or affected folder where data has been encrypted and sometimes changes Windows wallpaper. Somes types of ransomware will completely rename, encrypt or even scramble file names. Less obvious symptoms include adding or modifying registry entries and deletion of Shadow Volume Copies so that you cannot restore your files from before they had been encrypted but leaves the operating system working so the victim can pay the ransom. Further, when dealing with real ransomware, the cyber-criminals generally instruct their victims to contact them by email or website for decryption...they do not provide a phone number to call for assistance.
If there are no obvious extensions appended to your file names, no ransom notes, no demands of payment, your data is not actually encrypted, then you most likely are dealing with fake ransomware, Ranscam (Scam Ransomware), a screen locker (lock screen), a browser pop-up or web page indicating that "your computer is infected with ransomware", all your files are encrypted", "suspicious ransomware activity" or some other version of a Tech Support Scam. Typically the scammers provide a phone number for their victims to call for assistance.
For more information about how these scams work and resources to protect yourself, please read Beware of Phony Emails & Tech Support Scams...there are suggestions near the bottom for dealing with scams and a list of security scanning tools to use in case the usual methods do not resolve the problem or you allowed remote access into your computer.
If you need individual assistance with a malware infection, you should start a new topic in the Am I infected? What do I do? forum.