Q: How and where do I add the tickmark again?
A: Who could possibly know what you need more than yourself?
Q: Will I have to reset my Nexus 7?
A: Follow below steps, you'll be fine.
Q: If so are all the app I bought then lost?
A: As Flow said, you can recover them from Google Play store later on. If you feel not safe, use AppSaver to back them up.
Resetting your android phone
Before resetting your phone, you have to save your data (pics, music, movies, contacts...).
- If you have a google email account, saving your contacts is not
needed, you can just sync it to the cloud.
- Concerning your apps, as Flow said, you can recover them from Google Play store later on.
- Backing up pics, movies and music is necessary. I recommend you to back up your entire data storage, it contains apps private files, office documents, few stuff that you could easily forget to back up otherwise...
When those steps are done, you can safely reset your phone, it will be like a second life for it :-).