Help, Grub has corrupt display, Open Ubuntu or Win 10 eventually

[Charlie]

(I am not a expert on computers by any description. I don't talk the 
talk. I am sorry as I probably cannot present my need for help clearly 
to you: I installed win 10 and Ubuntu mate on my Dell Laptop using Grub. 
I did this a year or two and it has all worked fine until now. The start 
up screen display is corrupted, nothing but gray with some lines and 
signs that something is going on in the computer. By trial and error, I 
hit enter at a point as I did when selecting Ubuntu. I waited and Ubuntu 
opened and works fine. I repeated this idea and got win10 to open. I 
assume that Grub has a problem, I would sure like to repair this 
problem. I DO Not have a clue what to do or how to do it.

My plan: beg for your help and/or try to re-install GRUB to fix the 
problem. I don't desire to re-install Win10 and Ubuntu. (I am 80 years 
old and my memory is getting ragged.)

Question: Is a repair possible for GRUB? Is a re-install of Grub the 
solution and is there help toi do this?


Thank you for any help. I hope I have explained my need clearly.


Charlie