For starters, I'd suggest using a credit card right away. How many cards do you have? You only need to use one and let a small balance (say $10 or so) report every month. It can be a small purchase that you'd normally make anyway with cash, or use a monthly recurring bill like car insurance or your cable bill for example. This will register revolving credit use which will instantly give you 15-20 points.
Next, look into the Share Secure Loan technique. Search for it on this forum. Implementing that when you have no other open installment loans will yield you another 25-30 points instantly.
So, you're looking at 40-50 points by doing both of these things above which are incredibly simple. That will get you back to near 800 again.
Are you sure there are no negative items at all on your credit report? Even a minor 30 day late payment coudl take you from a near 800 score down to the low-mid 700's. Just make sure there's nothing on there that's holding you back. It does seem odd to me that your scores took a big hit suddenly. To me that suggests the introduction of a negative item. It could be something you're unaware of. I'd just double check that, just to be safe. Do you use any monthly monitoring services? Something like Credit Karma is free and will allow you to see both your TU and EQ reports which will get updated weekly. Or, for $1.00, you can get your 3B (TU, EQ, EX) FICO 08 scores from Credit Check Total so that way you can see your reports from every bureau as information can sometimes differ between them.
