my imac won’t start up stops on apple logo and 1/4 line

I would look at the current M4 based Mac minis. For about $1300-$1400 you can have a brand new machine that will run circles around your 2015. The configuration I would recommend is:

• 16GB RAM
• 1TB SSD

That comes to about $999, add an external display (a very nice 27" display) will be about $300, you can find many on Amazon, your local Costco, local Office Supply Store, Best Buy, etc.... You can use the keyboard and mouse from the old iMac and you are in business. To migrate all of your data from the iMac to the new machine simply follow the steps in Setup Assistant and Migration Assistant.

By the way, I'm in my 70's and use my iPad too. Where the iPad gets the most use is when I want to travel or just be mobile. Last year I bought the 11" iPad Air with 256GB of storage and a third party keyboard and pen, love the pen however the keyboard just so-so. My current Mac does most of the "heavy lifting."

“Not that old” is subjective, what is the model year of the machine? Also, I’d suggest calling AppleCare, if you are in the USA that is 800 275 2273 and getting a tech on the line. Don’t go back to BBY, while they are considered Apple Authorized Service Provider ‘s I wouldn’t trust them with my Mac. My tv, okay but my Mac NO WAY!!!

If your Mac starts up to an Apple logo or progress bar - Apple Support

In short, it has suffered a hardware failure. If the machine is a 2017 or earlier Mac then it's time to replace it. It does NOT make sense to spend money on a 8+ year old Mac.