Your computer has frozen. Sometimes it's on a page that is demanding money so they will give you control of your computer again. Sometimes it just dies, and you can not get it going again for love or money. Sometimes it's just running slowly and sometimes it completely crashes.
Most of the time we will say we have a virus in our computer and much like the ones we can get in our bodies, these viruses attack our computer's hard disk space or central processing unit which makes it very hard to fix.
By definition a computer virus is a malicious software program that replicates itself by modifying other programs and inserting its own code. That is, it gets into your computer and then takes over your programs sometimes leaving them not functioning at all.
Those who release these viruses are using social engineering deceptions to exploit the vulnerabilities that systems have. Sometimes, even if you have a security system on your computer the virus can circumvent it and still infect it. Viruses cost billions of dollars a year in economic damage because the damage they cause is so great. They are targeted for profit or political messages or sabotage. Sometimes the people who create them are just in it for the fun of watching users suffer, and suffer they do.
On the other hand, malware is malicious software, things like ransomware, worms, Trojan horses and spyware. They, thankfully, do not interfere with the hard disk space or take over control of your computer, although they are still annoying to have and to deal with.
There are many places that deal with removing malware from your computer and even programs you can download that will scan and get rid of them from your system. Now that we know more about these things, security systems have pre-programmed scans that take place at an allotted time to scan the computer and alert you to any problems before they get too big to handle. Using an anti virus program and your security you are in a good place to keep your computer running well. Sure at times it goes slow and sometimes you need professional help and luckily there are those who deal with computers on a daily basis and know what they are doing.
In the meantime, back up anything important so you do not lose it if the next big virus attacks your computer.