I do missions they don't give me exp, I kill moob they don't give me exp I've been killing moob for 12 hours and doing missions and I'm still at level 23 ... I helped my brother who was level 19 and now he's 23 almost 24 and I'm still killing him same for 12 hours, additionally he sees the bar experician I do not the moob or missions give me EXP
there is a NPC which allow to stop gaining exp. Maybe you have talked with him as mistake.
So go talk to him to start gaining exp again.
There is 1 in Orgrimmar and 1 in Stormwind:
Ally: Behsten
horde: Slahtz
(click on the name to see their position)