Missing one of your deployed fleet unit? From a total of 5 fleet it rolled back to 4?
This could be caused by your National Policy.
When you reset your National Policy, this will also remove all the benefits you've gained from those sub-policies.
To obtain the +1 deployed fleet unit, you need to unlock either of these policies.
- Liberalism > Volunteer Army
- Despotism > Royal Bodyguards
More Tips and Hints...
National Policy & The Importance of Resetting