Have only had my new Samsung Galaxy Ace GT-S5830i one week and while playing "Flow Free" game downloaded from Play Store my phone suddenly switched off and from then on constant "Force Close" messages appeared. I have no access to my phone. How can I get my phone back?
You could try to boot your Ace into "safe mode", as described e.g. in the article SAFE MODE On Samsung GALAXY ACE:
Now that you are in Safe Mode you can go and remove any buggy apps that you don't want to keep. To exit safe mode after your cleanup:
If that does not work out, there's always the factory reset. But be aware that this removes all your data plus the apps you installed yourself!
How to perform a factory reset on the Ace is e.g. described at hard-reset.com:
Press and hold the Home plus Power buttons simultaneously, or try Volume-Up + Home + Power together.
Of course there's also a menu for this -- but as you wrote you have no longer access to your phone, above key combinations are the only things left working.