There are three common ways in which this can happen, and all three of these scenarios can be equally crippling and put your system into a state where it seems to be frozen. None of these situations are harmful to your system (ie. they make it slow until the problem is resolved, but they don’t damage anything).
- A program is monopolizing the cpu.
A program is using all of the cpu cycles, blocking access to the cpu to other programs. This may be intentional (programs that do heavy processing) or accidental (programs get stuck repeating something over and over).
- You’re nearly out of physical memory.
You are either running too many programs, or programs that use too much memory. Your physical memory is almost entirely exhausted, and the running programs are using the harddisk as fallback memory, which is very slow.
- A program is doing heavy I/O.
You may be copying a large file, for instance. The program that is doing the copying is requesting lots of data from the harddrive, but while it’s doing this the cpu is actually waiting for this data to be read from the harddrive, blocking access to other programs.