Mac Mini does not come out of sleep mode

Hello, I have Mac mini which had Mac OS X 10.4 operating system on it. The Mac mini used to work fine with it. I recently updated the operating system on the Mac mini to Mac OS X Snow Leopard. After updating the operating system on Mac Mini I am facing a major issue. Once it is put in sleep mode Mac Mini does not come out of sleep mode after the operating system upgrade. Can operating system upgrade create such problem? If anybody is facing similar problem then please share some information about the same. Thanks in Advance.

By looking at the problem, it seems like an some hardware problem in the Macmini. Once the computer is put to sleep mode, to bring it back to normal working condition keyboard or mouse is used. It is possible that when the Mac mini is put to sleep mode keyboard and mouse are getting disabled some how and hence are not able to bring the mini back to working state. Try connecting a USB keyboard to the mini and then put it to sleep. Check if you are able to bring the mini back from the sleep mode by using the USB keyboard.

Many of the problems of the Mac system are solved by performing a PRAM reset. Sometimes invalid entries get filled up in the system RAM of the computer. If the RAM of the mac system are flooded by invalid entries then it creates problems while normal functioning of the Mac system. To bring back the system to a normal mode, the invalid entries in the RAM has to be cleared. To clear these invalid entries, PRAM reset is done. To perform PRAM reset :-
1. Find and locate the following keys (Apple, Option, R and P)
2. Press and hold the following keys while pressing power button.
3. Keep the keys pressed till 3 startup chimes are heard.
4. As soon as 3rd startup chime is heard, release the keys and let the system boot normally.

The operating system upgrade can create lot of problems in a system if it is not installed properly. I think the operating system on your computer is not installed properly and hence you are facing the problem in sleep mode. If you have the install disk of the operating system which you are using then insert it in the drive and then boot the system from the drive. Installer will load and will give you options for installation. Either you can choose to format and install fresh copy of operating system or you can choose Install/ Archive mode which will enable you to use the installed applications from the previous operating system. Reinstalling the operating system will solve the Issue.

There must be some applications running in the background which are taking up lot of CPU usage. Whenever a computer is put to sleep mode, during waking process, CPU has to process some processes which brings the Mac mini out of the sleep mode safely. If the CPU is not free to process these processes then the Mac mini wont be able to come out of Sleep mode. Hence while putting the Mac mini in sleep mode, check for all the processes which are running in background and disable unwanted processes which are taking up unnecessary CPU and memory usage. To check the processes which are running in background, use Activity manager from Apple menu.

Follow these basic troubleshooting steps and tell me if they work.
1. Shut down the computer and disconnect it from all the power source.
2. Remove the battery.
3. Now press the power button and hold it for 30 seconds.
4. After 30 seconds release the button and let the system sit for 5 minutes.
5. After 5 minutes, replace the batteries and the power source.
6. Now start the computer and then see if it is able to come out of sleep mode.