Someone's fiddled with your selector code for that extension range, probably. Check your automated attendant selector codes for 1, 2 and probably 3 to be sure they are set for "direct extension dialing".
If they are properly set and still won't accept the user's input, power cycle the switch and try a few minutes later. If it still won't accept them, there's likely a DTMFR problem or line quality issue that needs looking at.