Why does UE4 keep crashing?

Hi,
I started creating a project on UE4 in February, the first few months it worked great. I was able to create an endless runner level and everything was brilliant. But now I am looking into designing virtual environments. I was initially doing this on my laptop (MacBook Air) while UE4 was stored on an external hard drive. I could not create the environments as the program and the hard drive kept crashing so I thought this was due to my laptop not having enough memory or storage. So I have recently ordered an iMac. It arrived today. I spent 4 hours designing an environment using the ‘time of day’ level and using sculpt tools to create a mountainous environment. My program was working fine but it has just crashed again and now everytime I try to open that level, it crashes and I am unable to access it. Why does this happen? and how can I prevent it happening in the future?

Thank you for your help