Which Node version are you running (> node -v)? Did you recently update Strapi?
In general, you should try to rebuild Strapi (> npm run build) and then start it again.
If that did not help, you may want to remove node_modules and package-lock.json. Then build strapi and try again to start it.
If that did not help, please provide more information (e.g. node version, command used to start strapi, active plugins).