It's likely a faulty LCD or inverter. While it's likely you will need to replace the LCD or inverter, it can also be the fault of a loose connection.
The first thing to do before doing anything to the laptop is to check for an image. These 2007-2008 MacBook Pros have 8600M GT GPU's that are known to fail. To do this, shine a flashlight on the display and check for a stable image that doesn't have any defects.
If there are defects, you have a motherboard problem; not an LCD or inverter problem. As far as data goes you can use a USB enclosure to retrieve it unless you used FileVault. If FileVault is used, you will need to source out a 2007-2008 machine with a good GPU or do a reflow to retrieve your data. FileVault is opt-in on many versions of MacOS, so it's very likely your drive isn't encrypted.
The first thing I recommend in cases like this is to take the computer apart and open the LCD assembly to check the cables for damage or a loose connection.
Try reconnecting the screen and see what happens. Follow this guide to access it: MacBook Pro 17" Models A1151 A1212 A1229 and A1261 Display Inverter Replacement
If you are still having problems, the inverter may be at fault. While it's likely a bad panel, the inverter will be cheaper then a new LCD so it's worth a shot. To replace this, follow the inverter guide I linked in the previous line. Just bear in mind this system is obsolete and it's very likely finding one will be difficult without spending a lot of time to see that happen.
If you continue to have problems, the LCD is faulty. These are very hard to come by today and tend to be expensive when they do, so it may not be worth replacing it. Pull your data off with an external monitor, verify you have all of your files and either remove and destroy the drive or erase it and use the system for the remaining parts (with the exception of the motherboard, LCD, inverter, keyboard and chassis components) and use the remaining parts on other systems. However if you still want to replace the LCD, here's the guide: MacBook Pro 17" Models A1151 A1212 A1229 and A1261 LCD Replacement