Hi,
Press and hold the power button operated for >10 seconds and then release to ensure that the PC has fully shutdown in case it is in a sleep or hibernate state.
Once it has shut down try turning it on and check if it starts OK
If it doesn't start and boot into the OS then try a power refresh and check if that resolves the problem.
* Turn off the PC and then disconnect the power to the PSU.
* Open the PC and remove the coin cell RTC battery from the motherboard. (Once the battery is out, check its voltage. If it is <2.5VDC, replace it. The battery model number is stamped on the battery itself - CR2032 perhaps).
* ''Press and hold the Power button for a full 30 seconds and then release it.''
* Reinsert the RTC battery into the battery holder on the motherboard (remember its orientation, usually +ve on top as marked on the battery)
* Reassemble the PC, reconnect the power to the PSU and turn it on and check it it works OK.
If it turns on OK there ''may'' be a message stating that the time and date are incorrect. This is normal as the BIOS has been reset. Once the D&T have been corrected the message won't appear the next time that the PC is started.