Try starting your phone with the sim card and memory card removed and see if that helps. Also make sure the battery isn't flat. If holding the power button reboot does not help the only other option you can try is to hard reset the phone. However, this means you will lose everything on your phone unless you have a previous backup.
To hard rreset your phone, press and hold “Volume Down + Camera Key + Menu Key” and press power button until your phone vibrates.
You should be able to reinstall all the paid apps from Ovi Store.
Crashes can be caused by anything, hardware fault, data corruption, faulty sim etc. etc. So it would be pure speculation at this stage.
You could try your local Nokia Care Point but one of of the first thing they try is the hard reset so you will lose your data anyway.
Oops! My mistake. The hard reset does wipe the E: but not the F: drive. I absentmindedly forgot the N8 has both mass storage and memory card.
The hard reset will return the phone to its out of the box state. A backup, if there is one, can be used to restore data. I presume you did backup your phone as advised before updating. You should be able to use the same backup now.
The hard reset will return the phone to its out of the box state. Any widgets which are part of Belle will remain. Any widgets you installed yourself will be lost but you should be able to download and reinstall them from the same source as previously.
There is no "safe mode" in symbian. It probably did crash for a reason but it is impossible to tell what the reason might be. Most care points will probably not bother finding out either. They will probably just reset or reflash your phone. If that does not work then it is probably hardware.
