hello Wilbus,
did you disconnect the battery while working? If not, you made a short circuit in the video line by connecting the lcd matrix. You probably burned the filter coil in the screen's power line (this still happens). The tablet will not start up with such a short circuit, in addition, it quickly drains the battery, producing some heat. If you don't know anything about microelectronics, find a good repair shop, you can't do it yourself. Usually, people with no experience, when connecting the screen, try to hit the video connector, move the plug to the left and right. Unfortunately, this sends a current of 16-18V to signal lines, which are rated for 1-5V. The effect is just like on your tablet. A tip for the future - always disconnect the power source when working with electrical and electronic equipment. In a good repair shop, the service understands such cases and does not try to finish the owner with large costs. The repair is light and fairly inexpensive, and should not exceed $ 20 if the screen has not been damaged.
* regards,