1) When this sort of thing happens, the best first response is to save, close and re-open the file.
2) Another useful technique is to go to Wireframe mode with all Layers visible, <Ctrl> A
to select everything, then copy and paste into a new TurboCAD window. Please note that I have occasionally had to make a Layer invisible because a corrupted object on that Layer was really screwing up everything.
3) While not as useful as it once was, killing TurboCAD and re-starting it may be of use. You may as well reboot your computer if you're going to try this.
4) The most powerful method of fixing things short of uninstalling and re-installing is to exit TurboCAD, delete the Built-In folder and restart TurboCAD. Instructions for this procedure are found here: http://forums.turbocad.com/index.php/topic,9797.0.html
TC Pro Platinum 2020, 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1