Have you checked that the chain tension adjuster is aligning correctly with the guide bar when reassembling.
Also have you actualy tried resetting the chain brake.
The front hand guard gets pushed forward if the saw kicks back up at you. It is also possible to trip the brake by catching guard when moving or working on saw.
Pull the guard back towards the saw (usualy a bit further that its normal position). Normaly you will hear a click when the brake resets. This should release the brake band from around the cluchdrum/sprocket allowing thecover to locate and the chain to turn.
If you have tried resetting the brake the only other thong I can think of (though unusual) is that if brake band was tripped when the cover was removed one (without the drum to hold them) one or more parts have come adrift or got damaged.
Hope this helps & good luck.
Robin S
September 2008