I think the liquid crystal display is probably degraded with age, or possibly damp. I don't think you will be able to repair it. I presume you are keeping it well charged. The batteries also degrade with age, and you can buy replacements.
This often happens with old devices such as calculators and cordless phones. Usually when they are old enough to be sent to recycling.
That Yale range is perfectly adequate for most homes, I used to use them, I preferred the HSA6400 which could be plugged into a phone socket and call you in the event of an alert. It is a direct swap for the 6200 panel. If you have a house crammed with gold jewellery or other portable valuables, a professional who knows how can defeat them, but the average teen crack head has neither the knowledge nor the tools.
If you are happy with it, keep an eye on Ebay. Used ones crop up quite often. You can get the instructions online if yours are lost. You can fit an extra siren, for example indoors upstairs where you will hear it, or at the back of the house, there is no limit to the number of sirens you can have, or extra sensors if you find a cheap seller. Alkaline batteries last a couple of years and are cheap. The later, modern models are not compatible.
If you ever need to install these, I find it a great advantage to lay out all the components on a table, with the siren wrapped in a blanket, to learn them in and test them. This is much easier than trying to fiddle with them once they have been screwed into place and you need to climb ladders.