Don’t let that bother you too much. I know it seems like a large blow, and to be fair it is for someone in your situation, but even with your modest SP, 500M ISK could be earned in a single day of playing. You do need some capital to make ISK, but not much, and if you stick with the game, 500M ISK will seem like peanuts, or at least small potatoes, in no time.
You have a couple options. First, if you really feel completely at sea, I would use one of the additional spots on your account to make a new character and run through the tutorial. It never hurts to have an alt, and running through the tutorial and perhaps the Sister’s of Eve epic arc will help get you orientated again.
Then you have to decide how to go forward. You could decide to sell your original character, which probably will get you 10B ISK or so. This sounds good, but it will take you many months to build up a viable character for ISK generation again, but often this journey is one of the most enjoyable parts of the game for new players. You can try out many things - industry, exploration, missions, incursions - and pick what you like, or even use it to fund some lowsec PvP as 10B will keep you in frigates forever.
The other option would be to then switch back to your original main, and perhaps use skill extractors to turn some of that SP into ISK. You could extract a few million SP, maybe from skills you aren’t using (like if you trained mining but don’t want to mine ever again) and sell them on the market for a couple billion ISK. That would allow you to resume L4 missions or incursions, or set up an industrial operation, or many other things that require a little ISK to get rolling.
It’s hard to say what to advise without knowing how much you know and how set you are on a long term goal. So, the TLDR advice is as Lily said above: your SP/character is worth something and you can access that value in multiple ways using character trading or skill extractors. Don’t fritter it away, but if you are really ISK-limited, don’t be afraid to use that to achieve your goals in the sandbox.