how to make my Sony Ericsson w900i fix after installing software on it!!!

hey guys i need help to fix my sony ericsson w900i phone after installing software on it......the thing is my phone was great when i used it for 2years but suddenly when i downloaded a theme and tried to use it, my phone switched off and it didn't turn on even though i tried........so a friend installed software for my phone but now my phone has a lot of problems coz i am not getting any incoming calls and neither can i call anyone also i cannot connect to the internet or anything even when i tried to use a java application lik mig33 still it does not connect ........so wat should i do.....and eventhough anyone tries to call me it says that my phone is switched off........i really need your help....plz help...and i also think the software that was installed might not be the correct one for my phone......but that just a thought......plz help.....