Hammer Editor Out of Resources
Editing levels with the Valve Hammer Editor can take a large amount of system resources, especially when browsing large amounts of models and textures. Increasing the amount of system memory (RAM) can improve performance, but crashes are often a sign of some type of hardware issue (bad memory, hard drive, power supply, etc.).
To avoid running out of resources on systems with insufficient RAM, avoid scrolling through the entire texture browser, as this will load large amounts of materials. You can also close and re-open the Hammer editor to free up resources. You may not be able to work on larger-scale maps on a system with low resources.
A third option is to download a small tool whose purpose is to clean up memory at regular interval or simply on demand. This will clean your memory by emptying it of all cached textures and models. You can type the expression "RAM Memory Optimizer Cleaner" in a search engine to find one.