How to unbrick G620S Ascend?

Hello. I am having some serious trouble with my mother's G620S, which had recently started to "restart" itself out of nowhere, almost once every 1 minute. Putting restart between marks because it didn't restart completely, basically it just showed the Huawei logo like the phone was shutting off and then returned to the lock screen, closing all apps. Battery didn't seem to be overheating so I didn't think that was the problem, and I decided to opt for a factory reset; but now the phone won't even go past boot (it has two loading screens when turning on: one that reads Huawei Ascend G620S and one that simply has the Huawei logo, and it gets stuck on the first one). What to do now? I can still access fastboot. I am very aware that this phone is old and that I should buy a new one but I don't have the money and this phone worked pretty well for my mom, so please, if you could give me some help I would be very thankful. Have a nice day, if you need more info I will give it to you. I only know that the Android version is 4.4, phone is non-rooted, and I was using a micro SD as "main" storage. I don't believe the latter created any problem although I removed it before factory resetting.