The hidden lines were made visible by turning on Hidden Geometry in the View menu.
Solid Inspector 2 is available from the Extension Warehouse and Solid Solver is available from the Sketchucation Extension Store.
Keep in mind that Solid Inspector can fix some of the issues with the model but you’re going to have to do a lot of the fixing manually. Take a close look at where the counter meets the wall near the tall windows/doors. There’s a gap in there that needs to be closed up.
If you draw this or another similar model, I would suggest using modeling practices that avoid the the internal geometry and stray lines from the get go. You should be able to create the entire model without ever needing to clean up internal geometry that prevents if from being a solid component/group. Doing so will speed up the entire process and reduce errors, too.