How does the shutdown cost money? Are people making overtime cause otehrs are calling in sick? Were contracts lost?
With no money flowing, most government employees aren't getting paid during.
Basically, if the government ain't paying, the payees ain't spending, and the people selling to them ain't making money.
Gov't contracts had to be put on hold unless the contractor was willing to proceed "at risk" -- meaning they have no guarantee the gov't will pay them for all the work they did. Those not willing to proceed may have to postpone hiring or even place people on leaves of absence while the money isn't coming in. just like above, the money isn't making its way into the economy because there's a dam upstream.
Imagine all the grocers, especially the small mom & pop stores, having to meet payroll and pay their bills if a large percentage of their customers are SNAP recipients. Once that money is halted, grocers in lower income areas might have problems with their monthly cashflow, threatening their business' very existence.
Those are high-level issues. At the micro level, you may have parents unable to pay tuition for their kids' college or private school, new car purchases that have to be put on hold even though their old car is costing lots to keep it running, Same for new appliances or major work on homes -- new roof or kitchen renovation. Then there are services the employees can't all afford without a paycheck, like their yard maintenance or house cleaning, and on and on. Each person who depends on the gov't money reaching their customers (gov't employees) will be impacted.
Then there's the $5 Billion we owe in back pay for gov't employees, which almost all will be paid for zero hours worked -- money for nothing.