This just happened to me. I collected day 27 bonus yesterday, and today I'm reset to "Day 1". No mystery prize for day 28, no 20K funds for day 29, and no 8 diamonds for day 30.
Had you collected the bonus every day? If you didn't collect three days that will happen. If you collected every day without fail then you should submit a trouble ticket.
Just to add to @61stMed's answer, missing days is the only way this could happen.
This is because the daily bonus resets every 30 days, not if you miss a day.
My daily bonus resets to day1 after day 27. Im not getting the last 3, sometimes 4 bonuses.
I have a question Dave which you surly can answer. Is it true you can possibly get a T6 in a Mistry Box on the daily sign in on the third day of each week?
Are you getting your bonus everyday? If you miss a day it will reset before you get all 30 bonuses.
Also Feb was only 28 day so it may have screwwed up the cycle.