Emily, first of all, as Stephen says, take the sim out, give the contacts a quick rub to get any dirt or grease off them, then put it back in and try again.
Next, dial *#06# this will give you your IMEI number, which should match the sticker in the back, if it doesn't, or it has changed to all zero's then pm me and I'll tell you how to sort it out.
If they are both okay then the next question is how long is it since you last used used your phone/sim, if it is used VERY rarely, ie not for months at a time, then it is possible that Voda have cut it off thinking it is no longer in use.
Is it contract or PAYG, if contract make sure you last Direct Debit was made or Voda may have cut it off for a missed payment, even if the funds were in your account at the time.
Where did you get the phone, if you bought it new then (usually) there is no problem, but if second-hand then it is possible that the original owner has either not paid the contract or reported it lost/missing/stolen in order to get a new one, in which case it may have been blocked/barred.
It could be a broken aerial or power amplifier chip in the phone, but without seeing it I can't tell.
Lastly it could just be a software failure, some phones are more prone to this than others and a simple reflash (reloading new software) can often cure this. If you let me know which phone it is I can tell you how likely it is that this is the problem and if it can be fixed quite quickly.
Hope this helps