Sounds stupid but have you plugged both the mouse and keyboard into the same USB ports they were in originally?
If you have plugged them into different USB ports than when it last worked then there is a chance they wont be recognised.
I'm not 100% on this but I have seen this happen before.....especially with Dell.
Every time you connect a device to a USB port it has to be installed....this is still true if you change which port it is connected to......if neither of them have previously been installed on the ports you have them plugged in to then this could be the case.
If you log-in via the keyboard you may then find the 'Found New Hardware' balloon pop-up as it finds the devices.......the num lock light should come on on your keyboard and your mouse should work soon after.
If you can't log-in via the keyboard try swapping the ports the devices are plugged into....and if you can remember where they plugged in before then even better.
Hopefully its as simple as that to fix.
Regards.