Failing to do a hard reset and failing to wait after flashing are the two most common NOOB errors that lead them to a world of unnecessary dd-wrt pain! This is not a minor optional step. The firmware writes information to the Nvram. This step clears that information. If you don't clear it properly, parts of the old information be present with the new firmware, which can make it not operate properly. Don't cut corners. Doing it before you upgrade can be very important; a hard reset is not just for after upgrades.

Linksys WRT54 GL:

Linksys wrt54 GL users please note that if flashing with tftp using dd-wrt firmware gives no results, original Linksys firmware from www.linksys.com is worth trying. If that works, do a hard reset and you can continue to flash with dd-wrt. In order to use the Standard firmware version, a MINI version MUST be used first.