LEDGER SYNCHRONIZATION PROBLEMS

Recently, there have been many reports on the Internet that people's funds have disappeared on their Ledger wallets. Also, many people reported that they were unable to send funds on their Ledger and that they were shown the following error message:

The reason for this might be that the Ledger API (blockchain gateway) is experiencing some network issues, leading to problems with synchronization. Here is their official announcement:
https://support.ledgerwallet.com/hc/en-us/articles/115005198605-Synchronization-error-message

Here you find further information about this issue on Reddit:
https://www.reddit.com/r/CryptoCurrency/comments/7scwsw/just_so_you_guys_know_there_has_been_a_flood_of/
https://www.reddit.com/r/ledgerwallet/comments/7secj2/ledger_ns_couldnt_synchronize_data_followed_help/

When using a Ledger Nano S you are relying on Ledger's API infrastructure

After reading all this, it becomes clear that having your funds on a Ledger stick makes you completely dependent on Ledger's infrastructure. Also, you might depend on their support - which might not be the best situation to be in...

Preventive Measures

It might be a very good idea to know all your public addresses of the funds on your Ledger stick. This way, you can always use a blockchain explorer to examine whether your funds are still there.
Also, you should inform yourself about alternative ways to access your funds. This can be done with the 24 words seed you were initially provided with by your Ledger wallet.

Manually extracting all your keys from Ledger's 24 words seed phrase

On this website, you can manually generate all your keys out of your 24 word seed. You should be clear however, that you are exposing your private key. I certainly would not do that thoughtlessly. Your funds could be stolen if you have any malware installed on your computer.
https://www.ledgerwallet.com/support/bip39-standalone.html

Also there are other wallets where you could import your Ledger funds via your 24 words mnemonic seed phrase: https://support.ledgerwallet.com/hc/en-us/articles/115005297709

How to check the status of Ledger's infrastructure

There is a website that displays if Ledger is experiencing any problems at the moment. It is certainly a good idea to visit this website as a first reaction to some problems you are experiencing.
http://status.ledger.fr/

Good luck with your Ledger!

Remember: This is not financial advice.