|Laptop keyboards are a frequently replaced item and relatively inexpensive. |
Search for: Acer Aspire 5100-5022 replacement keyboard or similar.
e.g. $25 plus shipping
Acer doesn't seem to have Service manuals available for any of their laptop models on their web sites, but there are instructions for how to replace a keyboard on the web.
In the mean time, see your User manual for your model - you can use a USB connected keyboard.