Most of the grammar sources that I have access to tell me that 場合 can be preceded by the 辞書形 or the た形, but none of them explain the semantic difference between the two uses.
This is clearly not a simple matter of time or completion, because the following sentence is given in みんなの日本語:
Here, the losing of the card has not yet happened or been completed. Various native speakers have assured me that the same sentence would be valid with なくす instead of なくした but exactly what the difference in meaning would be is not at all clear to me.
How does the use of the た形 and 辞書形 affect the meaning of 場合 sentences of this kind?