Shit happens. Your clients can go out of business or get a new vendor. Your best employees can leave. Your technology can get out of vogue.
None of these are catastrophic failures. They can be overcome with retraining, marketing, getting new clients. Each of these take time. (And money).
Thats why you need to have a rainy day fund. This will keep your business afloat when things change.
In general, 4 months of runway are enough for a consulting business. (That is 4 month of all corporate expenses, payroll, lease, electricity and everything else)