use the search box at top of forum page to search: "system crashes","PC crashes", "computer freeze", "PC freeze". It takes a lot of time to run through many of the results but you will ad to your knowledge base. Some good responders here get tired of the same old questions and repeating answers.
Top Reasons for Occasional Freezes, No Reboot in Normal Mode:
1. Heat buildup, mobo shutdown.
Open up PC and look at processor heatsink/fan for dirt buildup-blockage; also vents on power supply; also case fan and case vents.
2. Hard drive shutdown.
Control Panel>power management. Check time set for inactivity before Hdd is powered down. Some PC's don't like this and will not power back up. Reset time for longer or none.
3. Too many things starting at startup.
start>run>msconfig>startup. Send this forum the list of items starting with Windows. We'll respond about what can be turned off and what may be spy related.
4. Bad application.
You need to note what you are running that may make this happen. Some apps will start to use up too much resource (ram). Some have become corrupted and need to be uninstalled, re-installed.
5. Hard drive problem.
at the bootup menu choose command prompt only:
C:/> scandisk/autofix/surface and look at the surface map for bad sectors.
hard drive needs defragged badly.
6. Virus, malware, pest interferring.
Get cleaners and clean.
7. Hardware problems.
Sound card, video card, modem, others. Usually a bad device will not allow boot at any time but there are some that act up after you are on for awhile. More info needed from you.