Yesterday iPhone said storage almost full so I deleted some things, avail space increased. Now phone is super slow and the messages (text) app doesn't open. It just shows a blank screen then after a minute it closes.
A reset will not cause any change of content, and is quite easy to do. Tap and hold the Home button and the On/Off button at the same time for approximately 10-15 seconds, until the Apple logo reappears. When you see the Apple logo, release both buttons and await restart.
OK, let's try this: Double tap the home button to expose the "Recently used" bar. Tap and hold one of the Icons until you see the Minus sign (-) in the top left corner. Tap each of the Minus signs to close all Apps. Try the messages app again after that.