Your balance can suddenly drop, and free spins disappear due to various reasons listed below:
You had a bonus and cancelled it. This will also remove any free spins you have left.
You deposited while having less than €10 or equivalent in another currency, and your bonus was cancelled automatically.
If you activate or deposit with any other bonus while having one already active, it will be cancelled automatically.
The bonus expired (3 days have passed), and the system automatically removed the whole bonus balance.
A withdrawal was requested.
For more details regarding your issue, please get in touch with customer support - they can explain what exactly happened to your account.