There is no "locked box". There never was. Congress started spending FICA taxes as soon as the entitlement was created and LBJ made it "legal" with a presidential authorization.
If by lock-box, you mean cash sitting in an account invested in nothing, then no there has never been a lock-box and for good reason. Most of the 2 trillion dollars in assets in the fund comes from interest earned on investments in treasuries bonds. Typically the fund earns 50 or 60 billion a year.
By law social security payroll taxes go to the treasury where benefits checks are issued. Any remaining funds must be deposited in the trust fund. Most of the funds assets are then invested in US treasury bonds and special issue treasury bonds. Any other use of the funds is illegal.
In most years there is sufficient payroll taxes coming in to pay all benefits, therefore as bonds mature, the proceeds are reinvested in order to maximize interest payments to the fund. Once in the 1980's and in 2010 and 2011, payroll taxes were insufficient to cover benefit payments therefor some benefits were paid from the fund. The amount was about 2% of the fund in 2011, less that the interest being earned.
Anyone that says "S.S. is bankrupt" or "there is nothing in the fund" is showing their ignorance.
S.S. has real problems. Very soon S.S. payroll taxes will not be enough to cover the benefit checks so we will start drawing enough from the trust fund to pay benefits. Sometime in the 2030's, not sure of the year, there will be no assets left in the fund. So what will happens to S.S? Benefits will continue to be paid out the S.S. payroll taxes, however the benefits will be reduced by about 25% possibly a bit more. This is the S.S. problem. It has little or nothing to do with the deficit. S.S is considered off budget, so there is no line item in budget to be cut.
Social Security history Frequently Asked Questions