First check your phone's SMS Messaging app to see if this pending message was successfully sent out by your phone (in some cases it may send out but has not yet updated on your computer or tablet). If this is the case, please refresh the computer web app (F5 in Windows, Command-R in Mac), or reload the tablet app.
If not, try the steps below and send another message after each step to see if this solved the problem:
(1) Launch MightyText on your Android Phone, and under Settings, select Unlink Phone. Then select your Google Account again, and tap "Complete Setup". Try sending a message again from your computer or tablet.
(2) Do you have any battery savers or task killer apps running on your phone? Or perhaps a "power saving mode" or "stamina mode?" Try disabling them (even temporarily) and send a message again from your computer or tablet.
(3) Make sure that your phone has a working and active data connection (either Mobile Data or WiFi). Try sending a message again.
(5) Make sure that you are running the latest version of the MightyText Phone App from the Google Play Store. If you aren't please update it, then try sending a message again from your computer/tablet.
(6) If your phone is on wi-fi, and you are on a corporate WiFi network (e.g. at your job), sometimes important internet ports are blocked on these networks. Ports 5228, 5229, and 5230 are needed by Google Play Services (which MightyText uses to communicate with your phone to send messages).
(7) If your phone is connected via wi-fi, try switching your phone from WiFi to your mobile carrier data connection and try sending a message again from the computer/tablet.
(8) Run this test tool on your computer: https://mightytext.net/test/roundtrip