In the short term, if you have or can borrow an external USB keyboard, plug it into a USB port and you'll find it much better for typing on than the on-screen keyboard. But if even that doesn't work you might have a software problem, though it's not something I've ever seen.
But my guess is that the keyboard ribbon has become disconnected from the connector on the motherboard. There's an excellent iFixit guide showing you how to remove the keyboard and re-connect the connectors - see ASUS D550MA-DS01 Keyboard Module Replacement
It may look daunting but you shouldn't find it hard, so long as you don't have 2 left thumbs. Nevertheless, make sure any valuable data is backed up before you start, just in case.
You'll need a small cross-head screwdriver. Take care not to loose the screws. Instead of a spudger you can use a guitar plectrum or a blunt kitchen knife so long as you hold it near the tip of the blade so you can't insert it into the case by more than a centimetre or so.
When you've released all the clips, lift the keyboard very gently and carefully from the front, so as not to strain the 3 ribbons linking it to the motherboard. The keyboard ribbon is the wide one. Gently flip up the black clamping bar on the connector to release the ribbon. If it doesn't flip up very easily you're doing it wrong. If you try and force it you may break it, then your keyboard really will be finished. Re-insert the ribbon, making quite sure it's in the connector as far as it'll go, then push the clamping bar down again. Test it before putting all those screws back. The clips should just snap back in place.