How can I fix my phone

How can I fix my Huawei p20 lite? - I've replaced the screen of my phone multiple times but have never replaced the frame. yesterday I replaced the screen and frame and while doing the test it worked. I then glued on the back and it would not turn on and had a flashing red light on the status indicator. I charged it up but to no avail. I then took the back off again and once again it worked flawlessly ( i did this a few times and the screen still would not turn on when the back was glued on). I then tried to glue on the back when the phone was on and it must have turned off itself because it will not work. i have taken it apart again but it will not turn on. When I plugged it in and pressed the power button the status indicator flashes red. I may have damaged the battery while fixing the phone and that may explain why it runs out of battery so quickly but I don't think it is related to it not turning on. If anyone has any ideas about what's wrong or a solution I would greatly appreciate if you could reply