Chances are your battery is dead. Over time they lose their ability to hold a charge. Think of the battery capacity as a jar. You can fill the jar with water, representing the amount of charge the battery can hold. Now every time you use the battery, you drop a marble in the glass. After a while, the jar is full of marbles. You can still add water (charge the battery) until it's full, but it only holds maybe 10% of the water it should be able to.
So yeah, your next step needs to be to replace the battery; it it hasn't been done since the tablet was new, it's almost certainly time.
I wasn't able to locate any iFixit-style step by step guides for that repair, but the YouTube video I found makes it obvious that battery replacement on your Fire tablet is actually pretty easy. No adhesives used, so all you need is a screwdriver and a plastic spudger (pry tool).
How to Replace Your Amazon Fire 7 (7th Generation) Battery - YouTube
You can help support iFixit's repair mission by purchasing the battery right here on this site.
Amazon Fire HD 7 (7th Gen) Battery
If you're more cost conscious, you can also find suitable replacement batteries on sites such as Amazon, eBay and AliExpress, among others.
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