Is there an air bag back in place?
If there is and you didn't follow procedures then it now needs re-programmed.
Disconecting the battery and waiting is a must.
Adding to this, you should disconnect the battery and wait for about half an hour (to be on the safe side) before disconnecting, if you were a paying member you would have been able to see the guide.
If you've replaced it with an aftermarket steering wheel, then it could be that the wiring is incorrect/a resistor is required (will mean that the airbag will not function), if it was an upgrade such as going from a standard MK2 PH2 steering wheel to a 1*2 wheel, it could be that you just need to disconnect them and reconnect them again.
When I was upgrading my steering wheel to a sport wheel I wired it up perfectly to the same airbag I had previously, but when I reconnected the battery and went for a drive the airbag light was illuminated...I took it all apart and reconnected the wiring in exactly the same way (after disconnecting the battery 30 mins in advance) and the light went off, could have just been that it wasn't getting the contact perfect.