Dear applicant,

There`s no need to panic, it is very common to have such issues. Keeping this in mind the examination authorities have made necessary arangements as well. You need to go to the official website of the examination you are talking about (WBJEE). There will be contact details on the home page itself, either on topmost part or on the bottom. Please contact the authorities. Make sure you have rest of the documents in place and ready to be used whenever necessary. They might ask for a written application as well. State a request and should not be difficult, although try not to delay. 

Hope it helps.

All the best for your future endeavours!