[When I open 'Get briefs' option.]

Don't worry, just make sure your gig is complete (has proper packages, detailed description, appropriate pricing, has a unique selling point, attractive gig images, etc.). My account was also in-active for about a year but things have picked up. You will get there, just deliver quality work. 

[When I open 'Get briefs' option.]

Buyer requests have been removed from Fiverr, and as for Briefs, it is not something you can open. If the system thinks you are a right match for a buyer, only then you will get a notification. 

[Gig Impression]

This has happened to a lot of sellers today, I am sure this is just a technical issue. This happened a few weeks back too, nothing to worry about it. 

[I decided to become a Seller Plus for 20 dollars a month, and my Impressions went to 0?]

If you are talking about your impressions showing '0' today then don't worry, its just a technical issue. It is showing zero impressions to a lot of sellers today, myself included. Also, I am sure Seller Plus will be a big help to you.