Try Reset Network Settings.
Reset Network Settings Settings>General>Reset>Tap Reset Network Settings. You may need to enter your wifi passcode back in afterwards.
If you're messaging a non iPhone, make sure MMS is toggled on in settlngs > Messages > Toggle MMS on.
I'm no expert, but I remember the times when a Data, not wifi, signal was required to send MMS. If that still holds true, then check your data. If not, resetting network settings like @Ledstplin suggest should work.