At the time of writing there are no reported problems at WhatsApp. However, WhatsApp can go down unexpectedly at any time of day or night. Click here to find out whether there are WhatsApp problems in your area.
If WhatsApp appears to be working correctly, it would appear that the problem is instead local to you. Below you'll find some suggested fixes for WhatsApp. Also see: New WhatsApp hoax surfaces.
Troubleshoot your internet connection
WhatsApp is an online service, which means you must have an active internet connection in order to use it. Fire up the browser on your smartphone and try to browse to a new web page, or open and refresh another online service such as Facebook or Twitter.
If your internet connection appears to be working, move on to the next tip. If you can't get online at all there are several things you can try.
If you're using Wi-Fi, first try toggling the connection on and off, or putting into and then removing the phone from Flight mode. Alternatively, you can try restarting your phone. Also see: How to use WhatsApp on the Web.
Also make sure your phone isn't dropping the Wi-Fi connection when it goes to sleep (go to Settings, Wi-Fi, choose Advanced from the Options menu, then ensure 'Keep Wi-Fi on during sleep' is set to Always).
If you're sure the broadband service itself is not down (downdetector.co.uk is a good place to check), you could also try restarting your wireless router.
If you're using mobile data then check your mobile data connection is not turned off in the Settings menu, and that you have sufficient signal. Also see: What do the two blue ticks mean in WhatsApp? and how to get WhatsApp on an iPad or Android tablet.
You should also check you haven't restricted background data usage for WhatsApp in the Data Usage menu, and that your APN settings are correctly configured. If you can use WhatsApp over Wi-Fi but not your mobile connection, it's likely that the APN settings are not configured to allow non-web traffic - check with your mobile operator.
Finally, check that you have not overstepped your mobile data limit.
WhatsApp gets new features all the time, which translates to a fair amount of app updates. Sometimes WhatsApp will not work unless you are running the latest version, and this is certainly the case if you have sideloaded the app on to a tablet or other Android device for which it is not available in Google Play. Also see: How to get WhatsApp on an iPad or Android tablet.
On an Android phone you should check for app updates by opening Google Play, tapping the three lines at the top left and choosing My apps, then checking the Installed tab for available updates.
On an iPhone open the App Store and tap the Updates icon at the bottom right of the screen. WhatsApp recommends also ensuring you are running the latest version of iOS.
Uninstall and reinstall the app
A final resort is to uninstall and then reinstall a fresh copy of WhatsApp.
Before you do so you will probably want to back up your WhatsApp conversations, which on an Android phone will require you to also be running the free Google Drive app.
Download this, sign in using your Google account details, then within WhatsApp choose Settings, Chats, Chat backup, then press the green Back up button.
When you reinstall WhatsApp it will automatically search Google Drive for a backed up copy of your conversations, so when it finds your backup allow it to restore it.
Read next: 5 best WhatsApp alternatives.