How can my calculator be fixed after water damage?

Water can indeed damage the innards of a calculator, because it promotes corrosion, and the corrosion may disable the calculator's electronic circuitry. The problem gets worse if you leave the batteries in the calculator while it's wet, because the battery voltage, however low, accelerates corrosion.

In your case, since more than a month has passed, the calculator may be irreparably damaged, but if you have an hour or so to spare you may try to take your calculator apart and carefully clean it. You can use denatured alcohol and a small brush to remove any crud (especially greenish crud which is a sure sign of corrosion). Pay particular attention to the battery compartment and the connectors that feed power from the batteries to the electronics. Wipe off excess alcohol with tissues, and let the parts sit for a while to dry out. Then reassemble the calculator, pop in a fresh set of batteries and see if it will come back to life.

Whenever water gets into electronic gadgets, remember the first thing to do is take out the batteries (if possible). Then you can either put it somewhere warm (but not too hot!) and dry and wait for it to dry out, or if that won't help, disassemble the gadget and clean it.

In case the liquid is not water but e.g. soft drinks, just letting it dry out won't work. Soft drinks contain sugar and sugar attracts water, so they tend not to dry out completely but become a sticky syrupy stuff. Disassembly and cleaning will be required to get rid of it.