I've had mine stuck at 95% complete, waiting for receipt acknowledgement since 1000 yesterday. IR's servers seem to have completely crapped out.
However if you don't owe any tax 'cos you're on PAYE, then take solace in the following from the IR's FAQ:
Fixed penalties
If your tax return has not been received by 1 February there will be an automatic £100 penalty.
There is a second fixed penalty of another £100 if your tax return is still outstanding six months later -
31 July.
You can appeal against the fixed penalties but you will have to show that you have just cause for missing the deadline.
For more information and a handy chart see 'Key Dates'.
Reduction of fixed penalties
If - when the tax return comes in - the correct amount of tax payable at the due date is less than the fixed penalties, then the penalties will be reduced to the amount of the tax that is due.