If you can see video on the screen with a flashlight, then you have a backlight problem. It could be the backlight voltage from the power supply, it could be a cable, or it could be the LEDs on the backlight strip.
Professional repair is always pricey. If you think you can DIY, see if you can find and download a service manual for your model. Then get a voltmeter, remove the back, find the backlight connector and measure the voltage. If none or low, disconnect the cable and measure it again on the PSU. If it is still the same, you have a problem on the PSU. If it is OK, you have an LED problem. PSU problems are likely bad solid state devices - Diodes, transistors, etc. It is easy (usually) to test diodes in circuit (the first suspect). Then transistors. Worst case, replace the whole PSU. Make sure it is the exact same part number as you have - OR it is specifically identified as an exact replacement part number.
Replacing the backlight LEDs is usually possible, but is more difficult.