I am using a demonstration copy. It seemed to work ok until today when my database would not open and the preferences window was blank. I then closed it and when I tried to reopen the program I got the colored pinwheel then a message that the program was unresponsive. I do have time left on my demonstration copy. Any ideas would be greatly appreciated and of course saving my data would be important. 
Richard.
Reinstalling the application and/or deleting the preferences should fix this. In addition, please check if anything has been logged to the system console.
Thanks. Will my data be intact with a reinstall? I am not familiar with the System Console. How do I check this? Richard.
Reinstalling the appliction or deleting the preferences doesnāt have any impact on the database. And the system console is accessible via Applications > Utilities > Console.app.
Richard, if you continue to have problems, send us a message (Help > Support).
In that message describe your problem. Let us know which DEVONthink application you are using, the version number, the model of Mac you are using and how much RAM you have. As Christian noted, copy the Console messages related to your attempt to open the database.
In System Preferences, please list the preference panes, if any, that are listed under the āOtherā category.
In Console you will also find (in the left column) LOG FILES > ~Library/Logs > CrashReporter. If thereās a recent log for your DEVONthink application, copy it and paste it into your message.