I always leave in the USB plug for my wireless mouse in the laptop, and the mouse has always been detected. The other day...

You are in luck! There is a section of the troubleshooting guide on iFixIt that addresses this exact problem! Try cleaning the insides of the keyboard before you consider replacing it entirely. Specific instructions can be found within the link.

This symptom could be due to a number of problems. In the past, the Acer Aspire 7736Z-4809 model in particular has faced issues with dead or malfunctioning motherboards - which can lead to secondary problems with the BIOS, battery and power supply. If this is the case then the motherboard needs to be replaced entirely. To make sure it IS your motherboard and not just the other components, check out the troubleshooting guide for ways to diagnose the problem.