Administrative Procedure (fine) - Temporary No-Parking signs

There should be a phone number and email on the ticket. Call and explain. They will probably ask you to submit full details by email. Write advising all the details, time you parked (ideally with some type of proof or good reason for parking there). Apologise profusely. You may be lucky. I was end of last year. But chances are that you will have to pay.

No. If you park on the street, it's always got a time limit on it.

The "point" is that you should not have abandoned your car. You should have checked it on a daily basis.

You are exceedingly fortunate to have a fine and not to have had your mechanical monster towed.

The signs only have to be put out 24 hours before the temporary prohibition starts (although in my experience it is often two or three days earlier).
If you are going to leave your car parked in a street or public car park for more than 24 hours, it is your responsibility to check regularly that no temporary restriction is about to be imposed.
As J says you are VERY lucky that your car wasn't towed away as that is the normal procedure for temporary restrictions. The cost of towing and subsequent storage would have added considerably to the cost of the fine so just pay it - and remember to check regularly in future.

If this happened in the last weeks, you can try the Covid excuse. Car parked outside, you couldn’t use it, you were not leaving house unless absolutely essential. Apologise and plead for understanding. Good luck.