jailbreaking should have no effect on wifi. How did you jailbreak? if you ran blackra1n, then installed cydia, all your hardware and os files should be intact. If your wifi isn't working, check that it's on under Settings->wifi, then check that you're connecting properly. If it still isn't working, you may have a hardware issue.
Kmisterk people are having wifi issues when using blackra1n
but the best thin u can do is restore to a custom firmware but u could try this an see if the wif works again
go to
settings
general
reset
reset network settings
I used blackrain to hacktivate/jailbreak/unlock and am having no issues with wifi. And i know of at least 4 or 5 others who have done the same as I and have no wifi issues.