UK visa application processed: no visa, no refusal letter

I just got back my passport. When I opened the package, it contained only my passport with absolutely nothing else in it, and not even a refusal letter was sent to my email (I checked my spam, main box, promotions etc.) They sent me only the following message:

Dear Applicant,

Your passport and supporting documentation are now ready for collection at the TLScontact UK Visa Application Centre. You can collect your documents in person or by a representative.
Collection in person

Please provide following:

Your Application Checklist
Your Original Photographic ID
Copy of Your Photographic ID

I don't know what to do. How can I find out what has happened?

Which category of visa have you applied for

Visitor Visa