Any ideas or can someone help me? I really do not know what to do
TL; DR: Your money was stolen. There is nothing you can do that has a significant chance of getting your $ 660 back.
The thief has access to the private keys that are stored in your wallet.
At some point in March 2019, a transaction was made and I did not do any transactions in March
One possibility is that someone has access to your wallet and has stolen your money.
A current problem was the purse Electrum.
However, if you have a PC wallet or access an online wallet from a PC, other malware on your PC may have stolen your passwords and used them to access your wallet, steal money, and eventually delete logs and data to cover their tracks.
I have entered my receiving address in (a) Block Chain (Explorer)
Most modern wallets are HD wallets that create a new delivery address for each transaction.
I've created new recipient addresses that I've read to show your available bitcoins sometimes,
I did not hear about it, it should be a mistake in a particular wallet. In general, I would be surprised if this helps.
If you find unexpected transactions in a Blockchain Explorer, it is unlikely that your Wallet software has an error. More likely, you are the victim of theft.
If your wallet misbehaves. One solution is to make a careful copy of your source words or private key (which you should already keep in a safe place) and then use it on a new computer (or phone, etc.) to download and install the wallet software just real trusted website and make a new duplicate wallet and let it completely sync. You do not have to delete your old wallet until the new one works completely.
It is safe for me to enter my address here
There are potential privacy issues if you use only one receive address. Everyone can see a list of all transactions made with this address. But it does not give anyone the opportunity to access your money.
You can also securely publish a transaction ID and the name and version number of your Wallet software (eg Bitcoin-Core 0.18.0, Electrum 3.3.4, …).
never Give someone your private key, seed set / recovery kit, or password for your wallet or wallet or account. If you change a previously-registered password, your wallet / account will not be backed up again.
Again, what I've read about creating a new address can sometimes show balances again: reddit.com/r/Iota/comments/6sim7o/
This advice refers to IOTA and not to Bitcoin.
I am sure that I have never used Bitcoin this year
Everything indicates that your Bitcoin was stolen.
I would not use this wallet anymore and I would not use this PC for a new wallet or financial activity. Even after zeroing the disks and reinstalling the operating system, I'm worried about BIOS malware, infected USB devices, and so on.