Welcome to the Nokia forums!
Is your screen frozen or is your device not turning on at all?
Are you able to start it up and access the homescreen?
I suggest you connect your device to Nokia Suite, create a back up and reinstall phone software.
If you are unable to get any response at all from your device please make sure it is turned off, remove the battery and wait 30 seconds before placing the battery back into the handset.
If you cannot get it to start please take your device to your local Nokia Care point where a technical expert will look at your device.
I hope this helps and that you manage to get your device up and running soon!