Does the whole computer lock up or just a particular application? If you're not sure, try force-quitting the suspicious app and see if that's the problem. The instructions can be found by reading OS X Lion: If you can’t quit an application.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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