Excel stops responding to keystrokes

For the past several weeks Excel just stops responding to keystrokes for no apparent reason. I'll go into a cell to enter data, when I try to enter anything alpha or numeric, it just doesn't respond. This has been happening every few minutes!!! In order to be able to enter data, I have to go into a different spreadsheet and click on any cell, then I go back into the original spreadsheet & can enter the data again. It also happens when I try to enter anything through the formula bar.

Why is this happening?? It is really messing with my efficiency! I usually enter data without looking up at the screen. Now after I enter each and every character I have to look up to make sure it was actually entered. It is really starting to drive me crazy!!

Excel 365 on a HP 15 Notebook PC running Windows 10 Home 64-bit version 1803. 

Intel(R) Core (TN) i5-4210U Cpu @ 1.70GHz 2.4 GHz, 8 GB RAM